The collar counties: DuPage, Lake, Will, Kane, McHenry
Standard Illinois rules, high tax rates, and quadrennial cycles that differ by township.
DuPage, Lake, Will, Kane and McHenry surround Cook and are frequently lumped in with it. For property tax purposes they are in the other Illinois, and the differences matter.
How they differ from Cook
| Collar counties | Cook | |
|---|---|---|
| Assessment level | 33⅓% | 10% residential |
| Equalisation factor | At or near 1.0 | 3.0355 |
| Reassessment | Quadrennial | Triennial |
| Who values | Township assessors | County Assessor |
| Appeal stages | One — Board of Review | Two |
The arithmetic is different
To recover the county's implied opinion of market value, divide your assessed value by 0.3333, not 0.10. An assessment of $110,000 implies about $330,000.
And a dollar of reduction is worth roughly a third of what it is worth in Cook, because there is no large multiplier applied on top. A $10,000 reduction is worth around $900 a year at a 9% composite rate, against about $2,277 in Cook.
We verified the ratio rather than taking it from statute: every DuPage parcel in our data carrying both an assessed and a market figure returns exactly 0.3333.
One stage, so file carefully
There is no Assessor-level appeal that cannot raise your assessment. You go to the Board of Review, and the Board can raise as well as lower.
In practice increases are rare for residential property and usually follow evidence the appellant supplied — most often a recent purchase price above the implied value. But the free first bite Cook offers does not exist here, so do the arithmetic before filing rather than after.
Talk to the township assessor first
This is the practical advantage of the collar counties. Township assessors run small offices covering a defined area, and they are usually reachable. A factual error — a wrong floor area, a demolished garage, a basement recorded as finished — can often be corrected directly, without a formal appeal at all.
That route barely exists in a county of 1.86 million parcels. Use it.
The data problem
Cook publishes its entire assessment roll as open data, which is why detailed analysis of Cook is possible and analysis of the collar counties is harder. Collar county parcel searches generally serve one property at a time.
For a uniformity argument that is workable but slower: you look up neighbours individually rather than querying a dataset. Budget an evening rather than an hour, and use the PIN structure to walk along your own street.
What is the same
The law. Uniformity, fair cash value, the grounds of appeal and the evidence that works are identical statewide. Only the arithmetic, the calendar and the offices change.